1. Field of the Invention
The present invention relates to an optical network system in which each transmission path consists of a pair of optical fibers for transmitting signals in opposite directions to each other, and to an optical add/drop device used in such an optical network system.
2. Description of the Related Art
Conventionally, optical network systems have been developed to be mainly used for long-distance transmission. The optical network systems are designed in such a way that the distance between optical nodes provided on each transmission path is generally, for example, 100 km. Accordingly, as shown in FIG. 1, optical amplifiers are provided between respective optical nodes (or at the respective optical nodes) for amplifying optical signal in such optical network systems.
The optical network system shown in FIG. 1 is a WDM (Wavelength Division Multiplex) system in which signals at a plurality of wavelengths (λ1 through λ4) are multiplexed and transmitted and in which four optical nodes and four WDM optical amplifiers are provided on the transmission path. In this configuration, each optical node adds and drops, to and from the main transmission path, the light with the wavelength corresponding to the optical node.
In recent years, optical networks have also been realized for use in urban areas with relatively short transmission distances. These networks are often called optical metro access networks, and the distance between optical nodes is generally 10 km. This means that the transmission loss between optical nodes in these networks is relatively small. Accordingly, the optical metro access network can be configured so that one optical amplifier is provided for a plurality of optical nodes, as shown in FIG. 2, in order to reduce costs. In the example shown in FIG. 2, one WDM optical amplifier is provided for the four optical nodes.
However, if the optical signals are added to the main transmission path through optical nodes with the same optical power, the power will be different among the respective wavelengths in the multiplexed WDM light. Here, optical levels at the input port of the optical amplifier used in a ring optical network system including four optical nodes and an optical amplifier provided between optical node (#1) and optical node (#4) are discussed. The optical signals are transmitted in the counterclockwise direction in the above configuration in FIG. 3.
In this case, the distances between the respective optical nodes and the optical amplifiers are different from each other. Thus, in the example in FIG. 3, when the optical signals with equal power are added to the main transmission path through the respective optical nodes, the optical power level of the signal added through optical node (#1) is the minimum while the optical power level of the signal added through optical node (#4) is the maximum at the input port of the optical amplifier. In other words, the levels of the respective optical signals are very different from one another. This causes the problems listed below.    (1) The dynamic range of the receiver has to be broadened.    (2) Crosstalk is caused when dropping signals in the respective optical nodes.    (3) Signals deteriorate due to gain saturation in the WDM optical amplifier.
Patent Document 1 (Japanese Patent Application Publication No. 2004-15729 (FIGS. 2 and 3, paragraphs 0037 through 0041, and Abstract)) discloses a technique for solving these problems. Japanese Patent Application Publication No. 2004-15729 (FIGS. 2 and 3, paragraphs 0037 through 0041, and Abstract) In the optical network system described in Patent Document 1, variable optical attenuators (VOA) are provided at the respective optical nodes for adjusting the optical power of each of the added signals, as shown in FIG. 4. The variable optical attenuators provided at the respective optical nodes are respectively adjusted such that the levels of the signals are equal to one another at the input port of the optical amplifier provided on the transmission path. As a result, all of the wavelengths of the WDM light including the signals added through the optical nodes have a generally equal power.
An O-UPSR (Optical Unidirectional Path Switched Ring) is known to be one of several configurations that improves the reliability of optical networks. In the O-UPSR, duplex optical transmission paths are used; in these paths, a pair of optical fibers transmits the same data in opposite directions.
However, the technique of making the optical power levels of the signals added through respective optical nodes equal to one another on the transmission path in a system that transmits signals in opposite directions through a pair of optical fibers is not known. Accordingly, in this case it is assumed that each optical node used in such a system has a configuration shown in FIG. 6.
In the optical node shown in FIG. 6, optical signal output from the transmitter (E/O) is split by a bidirectional splitting coupler. The split ratio is 1:1. The output of the bidirectional splitting coupler is a pair of signals that are added to the transmission path in the clockwise direction and the counterclockwise direction, and the optical power of each of the signals is adjusted by the variable optical attenuators (VOA). The adjustment by the variable optical attenuators for the transmission paths in the clockwise direction and in the counterclockwise direction is performed on the basis of the method described in Patent Document 1, being explained with reference to FIG. 4.
Via the above configuration, it is essentially possible to make the optical power levels of the respective wavelengths in the WDM light including signals added through the optical nodes equal to one another on the transmission paths, respectively in the clockwise direction and in the counterclockwise direction. However, in this configuration, the optical power of each of the signals added through the respective optical nodes is decreased. This causes the problems listed below.    (1) Transmission distance is shortened.    (2) The number of WDM optical amplifiers has to be increased.    (3) The number of optical nodes that can be provided on the network is decreased.
As described above, the technique of making the optical power of the respective wavelengths in the WDM light including signals added through optical nodes equal to one another in optical network systems using a pair of transmission paths that transmit signals in opposite directions has not been conventionally established. Also, when the optical power levels at the respective wavelengths in the WDM light are made to be equal to one another by combining the prior art configuration, the optical power of the signals is decreased.